The present invention relates to stainless steel alloys modified to withstand radiation damage attributable to fast neutrons. In a method sense, the invention to be described provides an alloy design formula wherein existing alloys may be modified or new alloys may be synthesized which exhibit enhanced resistance to density and dimensional changes resulting from exposure to fast neutrons at high temperature, i.e., greater than about 300.degree. C. In still another sense, the invention resides in stainless steel clad nuclear fuel elements intended for use in fast neutron environments wherein the stainless steel cladding is an austenitic stainless steel containing void suppressing concentrations of Si and Ti and the nuclear fuel is an oxide such as UO.sub.2, a nitride such as UN or U.sub.2 N.sub.3, a carbide of uranium (such as UC or UC.sub.2) or mixed with an oxide, nitride, or carbide of Pu or Th.
The core components of a thermal or fast nuclear reactor are known to undergo a variety of stresses during their service life. For example, the fuel cladding will experience thermal and mechanical stress due to such factors as fission gas pressure, fuel-cladding interactions, and differential thermal swelling due to development of thermal gradients in the core. Nuclear transmutations, particularly (n,.alpha.) reactions, play an important role in radiation behavior of alloys, such as stainless steels. Experience with stainless steel has shown that the combination of thermal and irradiation effects lead to hardening and embrittlement of fuel core materials and supporting structural elements.
As materials research extended to the study of irradiation effects in fast breeder reactors, a new radiation phenomenon was discovered. In 1967, Cawthorne and Fulton of Dounreay Experimental Reactor Establishment, UKEA, reported that stainless steel fuel cladding exposed to neutrons developed extensive internal porosity in the form of small cavities or voids. The British finding stimulated activity in the field of irradiation damage as reported in "Radiation-Induced Voids in Metals" dated April 1972, a publication of U.S. Atomic Energy Commission, Office of Information Services.
Irradiation-induced swelling results from precipitation of vacancies into voids and interstitials into dislocation loops. Creation of the vacancies and interstitials is the result of collision between a neutron and a lattice atom. In such a collision, a portion of the neutron energy is imparted to a lattice atom sufficient to tear away from its lattice site. The result of this collision is the production of vacant sites and the atoms rejected from their former positions end up in interstitial sites. The dominant features of void swelling can be described as a phenomenon characterized in that it occurs in a fast neutron environment at elevated temperatures in the range 350.degree.-600.degree. C. Swelling increases approximately linearly with fluence after a threshold dose is exceeded. The swelling does not appear to saturate. As voids form, density decreases, and hence volume increases. In stainless steels the amount of swelling varies with temperature, with most swelling occuring between 400.degree.-600.degree. C. Operating temperatures of LMFBR cladding and structural components are 350.degree. to about 700.degree. C. and thus encompass the temperature of maximum swelling of 300 series stainless steels. Reactor design engineers who have studied the economical implications involved with the phenomenon of radiation-induced swelling have estimated that cost savings of the order of billions of dollars could be realized if void swelling could be reduced only a few percent in the 300 series stainless steel alloys contemplated for use as fuel cladding in fast breeder reactors under design or contemplated for construction.